LED retrofit lamps such as MR16, PAR38, Class A and down light are increasingly finding their way of replacing traditional illuminating devices such as incandescent and fluorescent lamps because such LED retrofit lamps are more energy-efficient and have smaller sizes and longer lifetime. With the technological development, LED package itself can achieve higher efficiency such as 160 lm/W for cold white and 100 lm/W for warm white and have a long lifetime up to 50,000 hours, but when LEDs are integrated into a retrofit lamp together with an LED driver, a thermal management device and an optical component, the efficiency and life of the retrofit lamp are highly dependent on how to design the driver, heat radiation device and optical component. In an LED, the consumed electrical power is converted to heat rather than light. According to the U.S. Department of Energy, about 75% to 85% of energy used to drive LEDs is converted to heat, and the heat must be conducted from the LED chip to the underlying printed circuit board and heat radiation device. If the heat is not removed in time, excess heat can not only reduce an LED's light output and produces a color shift in the short term, but also shorten the lifetime of the LED in the long term.
The primary path of heat transfer in an LED is usually from the junction to the outside of the package. The package level thermal management is provided for LED device manufacturing to minimize the thermal resistance from the junction to the outside of the package. The essence of thermal management design of a LED lamp is transferring the heat efficiently from the LED package to the ambient surroundings. First of all, a secure and thermally efficient bond must be provided between the package slug and the circuit board. The thermal connection typically runs through a metal core PCB. Heat is typically conducted through this PCB to an external heat radiation device. The Heat radiation device provides a path for transferring heat from the LED package to the ambient surroundings in three ways: conduction (heat is transferred from one solid to another), convection (heat is transferred from a solid to a moving fluid, and for most LED applications, the fluid will be air), and radiation (heat is transferred between two objects with different surface temperatures through electromagnetic waves). The heat conduction through the heat radiation device itself is associated with the following factors: thermal conductivity of heat radiation device materials (k), conduction area (A) and length (L) (Fourier law: Q=k×A×ΔT/L). For a certain amount of heat (Q) passing through the heat radiation device, the higher the thermal conductivity or the larger the conduction area or the smaller the conduction length is, the smaller the temperature rise (ΔT) within the heat radiation device is. The heat convection from the heat radiation device to the ambient surroundings is associated with the following factors: surface area (A) and local convection heat transfer coefficient (h) (Newton law: Q=h×A×ΔT), which depends on the size and geometry shape of the heat radiation device. The heat radiation from the heat radiation device surface to the ambient surroundings is associated with the following factors: surface area (A) and surface emissivity (ε) (Stefan-Boltzmann law: Q=ε×A×σ×ΔT4, where σ is the Stefan-Boltzmann constant). Therefore, the overall heat dissipation capability of the heat radiation device relies on the heat radiation device materials, the heat radiation device size and geometry shape and the surface treatment of the heat radiation device surface. For the heat radiation device materials, aluminum is a widely used heat radiation device material nowadays because of its high thermal conductivity and relatively low cost, while ceramic and thermally conductive plastic are also used as the heat radiation device material in some applications and designs.
For most of the current heat radiation devices for LED retrofit lamps, they are usually made of aluminum, and in some patents, a combination of high thermally conductive materials and thermally conductive plastic is used to manufacture the heat radiation device, mainly for the purpose of reducing weight and complex shape manufacturing.
Patent Document WO 2009/115512 A1 discloses a heat radiation device and a process for producing the heat radiation device, wherein said heat radiation device comprises a plastic body made of a thermally conductive plastic material comprising expanded graphite in an amount of at least 20 wt. %, relative to the total weight of the thermally conductive plastic material and has an in-plane thermal conductivity of at least 7.5 W/m/K. The heat radiation device can be produced by injection-moulding the thermally conductive plastic material, optionally followed by applying a coating layer. The heat radiation device and the heat source are assembled together by being thermally connected to each other.
Patent Document US 2003/0183379 A1 discloses a composite heat radiation device utilizing a high thermally conductive base and low thermally conductive fins. The base is preferably made of an anisotropic graphite material, and the fins are preferably made of a thermally conductive plastic material. In the case of a low profile heat radiation device, the fin height is no greater than 15 mm. This composite construction provides superior cooling effect yet lighter weight as compared to a conventional all-aluminum heat radiation device of the same dimension.
Patent Document US 2007/0272400 A1 discloses a heat radiation device having tapered geometry that improves passive cooling efficiency. The tapered geometry between heat radiation device heat dissipation elements decreases resistance to ratification of passively flowing cooling gas upon heating. Thus, the tapered heat radiation device elements results in high velocity of gas flow and increased cooling efficiency of the heat radiation device. Optionally, the heat radiation device is made from a thermally conductive polymer allowing the heat radiation device to be created in complex shapes using injection moulding.
Ceramic may also be used a thermally conductive material. Patent Document WO 2010/136985A1 discloses an illumination device comprising a light source arranged to generate light, and a carrier arranged to support the light source. Further, the carrier is arranged in thermal contact with the envelope and both the envelope and the carrier are made of a ceramic material. The disclosure is advantageous in that it provides an illumination device providing an effective heat transfer.
However, the drawback of the above disclosure lie in, when aluminum is used as a material for manufacturing the heat radiation device, the heat radiation device has a relatively large weight, and the shape design and manufacturing thereof are confined. The thermally conductive plastic material has a thermal conduction performance similar to aluminum in some applications and designs, and offers the benefits of lower weight, more design freedom and easier manufacturing, but the material cost is relatively high. Ceramic has relative good thermal conductivity but it is heavy and brittle.